In particular, the present invention relates to the problem that high-pressure and low-pressure discharge lamps require high voltages for starting. These voltages are generated by electronic ballasts with the aid of a series resonant circuit. In the case of electronic ballasts for two or more lamps, two or more parallel-connected load circuits are often used. In the case of electronic ballasts for two lamps, with reference to which the following invention is described by way of example, the bridge circuit therefore needs to be designed for twice the current than in the case of operation with a single discharge lamp. Generally, both load circuits do not have precisely the same resonant frequency. This can mean that one discharge lamp starts earlier than the other. If this happens, the current in the load circuit with the started discharge lamp is markedly lower. A starting controller which is provided in the electronic ballast continues to permit twice the current in comparison with the starting of a discharge lamp, however, in which case this current largely flows through the unloaded, started load circuit. This results in markedly higher starting voltages than is desirable in this circuit which is running off load.
One procedure for largely gaining control of this problem is known from EP 1 337 133 A2. This application relates to an operating circuit for a low-pressure gas discharge lamp, in which a digital controller is designed in such a way that it triggers shutdown operations of a safety shutdown device for excessive currents by gradually lowering the operating frequency in the starting operation, in order thereafter to slightly increase the operating frequency in a renewed starting attempt again. An attempt is thereby made to achieve overall starting by repeated pulse-like starting operations until shutdown operations or else by a continuous starting operation at a minimum frequency, at which no shutdown operation occurs.
However, problems associated with the solution are the fact that only the current is controlled there and a shutdown operation is triggered if this current is too high. Then, the starting operation is repeated. If this system is used in the case of electronic ballasts for two lamps, the abovementioned problems result. In order to prevent this, the load circuits and in particular the dielectric strength of the components, for example inductors, capacitors, diodes, have been selected to be correspondingly high. This is associated with high costs and is therefore undesirable.